Slideshow Block: additional event listeners for init

#### Is your feature request related to a problem? Please describe.

The [initialization function responsible for making slideshow blocks work](https://github.com/Automattic/jetpack/blob/master/projects/plugins/jetpack/extensions/blocks/slideshow/view.js#L20) is run on `DOMContentLoaded`. For themes whose posts are dynamically rendered, this may be too early.

Some events also trigger a re-render of the post, e.g. for o2-powered themes, pinning or unpinning a post.

Slideshow blocks in these scenarios don't work as expected.

#### Describe the solution you'd like

In addition to listening for `DOMContentLoaded` (via `domReady`) can we add another event listener for triggering the init code when a post is rendered? Maybe something like a `post-rendered` event, which themes/plugins can trigger when posts are dynamically rendered.

#### Describe alternatives you've considered

2262-gh-Automattic/p2 is a workaround that re-evals the init script to make slideshow blocks work for `pub/p2020`.

#### Additional context

Start in projects/plugins/jetpack/extensions/blocks/slideshow/view.js at the initialization function linked in the issue, and review how domReady currently triggers it. Compare that flow with the 2262-gh-Automattic/p2 workaround and determine the post-rendered event contract. Done means slideshow blocks initialize correctly after dynamically rendered posts as well as on DOMContentLoaded.

Written by the indexing model from the issue text.